I think that this advice can be summed up as you should make other people feel involved and appreciated.
I saw another video that claimed that the way you gain confidence is to practice. This odd advice might not seem applicable to most things, but I think that it is. I would not be confident making a speech or in an awkward social situation, but I am pretty confident over a chessboard. However, if I had spent 47 years making speeches then I most likely would be reasonably competent at it.